Without going into too much detail, think of it this way. You have one liter of CH3 and one liter of H2. Now, because of the nature of gases, each liter contains exactly the same number of molecules. However, each molecule of CH3 is much heavier than each molecule of H2. So, even though you have equal volumes and equal number of molecules, you have a much greater mass in the liter of CH3.
When you burn the CH3, a greater total amount of energy may indeed be released in the reaction that that released by burning the H2 (I don't know one way or the other off the top of my head). However, you're talking about the adiabatic flame temperature, and you're not looking solely at the energy liberated. What you're considering when you talk about the adiabatic flame temperature is how that liberated energy is absorbed by the products of combustion and figuring out what the highest flame temperature would be if everything went perfectly during the combustion. So, when you consider that the CH3 will produce a much larger mass in its products of combustion than the H2 will, the CH3 can burn with a lower flame temperature than the H2 will even though the combustion reaction itself may liberate more energy because it has much more "stuff" to heat up.